Magnetrons are devices for generating high frequency power. In one type of magnetron, a cathode is surrounded by an anode in a generally coaxial arrangement. The anode includes resonant anode cavities which may, for example, be defined by a plurality of radially extending anode vanes or by some other configuration and the space between the cathode surface and the anode vane tips provides an interaction space. In operation, an electrical field is established between the anode and the cathode and a magnetic field is provided transverse to the electric field. Electrons emitted from the cathode are acted on by the electric and magnetic fields. Resonances build up in the anode cavities to produce a high frequency energy which is extracted from the anode space by a suitable coupling mechanism.
One known magnetron is schematically illustrated in longitudinal cross-section in FIG. 1. The magnetron includes a generally annular cavity 1 which coaxially surrounds a cathode 2 arranged along its longitudinal axis. An anode 3 comprises eight vanes 4 and the wall 5 of the cavity 1. Two magnetic pole pieces 6 and 7 are arranged at opposite ends the cathode 2 and are designed to produce a substantially axial field in the interaction region of the magnetron. A U-shaped piece 8 provides a return path for the magnetic flux. Sets of anode straps 9 are included to connect alternate ones of the vanes 4 to control the mode of resonance of the magnetron.